This invention relates to an improved anchor bolt and expansion shell for use in underground and open pit mining, tunneling and surface excavation.
In using rock bolts for mine roof and wall support and for similar purposes, the rock strata is bored to a suitable depth as determined by the rock formation, it being desirable to bore to a depth sufficient to include in the strata a relatively thick layer of relatively hard rock. In boring, sections of the bore may deviate somewhat, variations in diameter and other irregularities occur, and on inserting an expansion anchor into the bore, there may be tendency for the anchor shell to shift laterally or become somewhat misaligned from the general access of the bore and the bolt. Such action may be due to deviations in the bore or soft spots or irregularities in the bore at the particular location where expansion of the anchor shell is contemplated. As the shell segments are expanded away from the bolt, the guiding effect of the bolt, to hold the segments in line with the bolt is gradually lost. Frequently, the shell segments will shift laterally or become askew and with the application of tension to the bolt, failure of the shell will result.
In the prior art there have been disclosed numerous expansion shell assemblies, examples of which can be seen in the following patents: Canadian Patent Nos. 515,957 issued August 23, 1955 to Joseph B. Dempsey; 590,082 issued Jan. 5, 1962 to The Ohio Brass Co., 618, 342 issued Apr. 18, 1961 to Joseph B. Dempsey; 748,157 issued Dec. 13, 1966 to the Eastern Co.; 756,832 issued Apr. 18, 1967 to Rudolph Stutz; 782,802 issued Apr. 16, 1968 to Torque Tension Bolt Co. (Proprietary) Ltd.; 786,056 issued May 28, 1958 to Michell; 936,027 issued Oct. 30, 1973 to Richard H. Warner; 937,792 issued Dec. 4, 1973 to The Eastern Co.; 946,190 issued Apr. 30, 1974 to Huygmetaal N. V. and 1,153,225 issued Sept. 6, 1983 to Heinrich Sohnius; U.S. Pat. Nos. 2,988,950 issued Jan. 20, 1961 to J. b. Dempsey; 3,238,731 issued Mar. 8, 1966 to H. C. Seifert et al; 3,250,170 issued May 10, 1966 to N. H. Siegel; 3,306,051 issued Feb. 28, 1967 to G. H. Howlett; 3,528,253 issued Sept. 15, 1970 to L. J. Kovacs; 3,683,741 issued Aug. 15, 1972 to George H. Peat; 3,702,060 issued Nov. 7, 1972 to James Dean Cumming; 3,837,258 issued Sept. 24, 1974 to Chester I. Williams; 3,967,455 issued July 6, 1976 to John P. Conway; and 4,011,787 issued Mar. 15, 1977 to Lewis P. White et al.
Because of their length, lengths of up to 40 feet not being uncommon rock bolts are extremely heavy and a falling bolt can be not only dangerous for the worker, but also damaging to expensive mining or excavation equipment. It is thus absolutely necessary that an expansion anchor be as failsafe as possible, i.e. as sure and reliable as possible, and further, be capable of being quickly set in place. Generally a power tool is used to apply tension to the rock bolt. Thus, the forces acting upon the expansion anchor are tremendous. In the event the rock bolt assembly is not properly positioned within the bore hole, the forces are definitely sufficient to readily damage the anchor shell and render it inoperative. Aside form safety aspects and possible injury and damage to workers and equipment, respectively, there is also the problem that the end of the rock bolt and the expansion anchor must remain properly aligned once positioned within the bore hole and during entry so that the anchor will expand in the required way to ensure that the assembly is secured in place. With the bolts previously described, it seems that the failure rate because each bolt does not remain aligned with the expansion anchor can be as high as 15 percent. Thus, there is the additional problem of the expense of damage or wasted anchors.
In the expansion anchors, described previously, attempts have been made to ensure that the end of the rock bolt remains aligned with the top of the bail so that when tension is applied to the lower end of the bolt the bail is not broken. For example in Canadian Patent No. 618,342, there is described an expansion anchor shell having a U-shaped bail with an enlarged bight portion in which there is provided a circular aperture sized to receive the end of a rock bolt. The problem with this shell is that frequently during installation, dirt may the threads of the rock bolt and hence damage to the assembly can be caused, making it inoperative. In Canadian Patent No. 618,343 there is shown a flat knockout plug in the circular aperture of the enlarged bight. However, the flat surface does not ensure that the end of the rock bolt remains correctly aligned with the aperture and cocking of the U-shaped bail can readily occur. This can result in damage to the bail when tension is applied to the rock bolt via a power tool and consequently it becomes inoperative.
In Canadian Patent No. 1,153,225 there is described an anchor bolt expansion shell wherein the bail includes a cap member which incorporates a central dimple portion extending halfway in a direction towards the leaves with a peripheral line of weakness formed about the dimple portion. The difficulty with this structure is that the end of the bolt is frequently fouled and contact with the downwardly directed dimple portion is not so certain and hence expansion shell failure is substantially high than desirable and certainly within the 15 percent figure previously mentioned.